1. Field of the Invention
The present invention relates to linear actuators and, in particular, to a moving coil actuator that utilizes the interaction of axially-magnetized permanent magnets assembled in opposition to provide the desired air gap flux density.
2. Discussion of the Prior Art
U.S. Pat. No. 4,808,955 issued to Mikhail Godkin and Jack Kimble on Feb. 28, 1989, and commonly assigned herewith to BEI Electronics, Inc., discloses a moving coil linear actuator that utilizes the interaction of magnetic circuits to provide a desired air gap flux density.
As shown in FIG. 1, the Godkin/Kimble linear actuator 10 includes a cylindrical core 12 and a shell 14 which is disposed around the core 12 to define an annular space between the inner wall of the shell 14 and the outer wall of the core 12. A non-magnetic spacer 16 is mounted in the annular space, at the actuator's longitudinal midpoint, to define a shell and core arrangement for back-to-back linear actuators. A first set of magnets 22, 24, 26, 28 is mounted within an annular cavity 18 of what is illustrated in FIG. 1 as the "right-hand" actuator. (Although one skilled in the art will understand that each magnet shown in FIGS. 1-3 has both a North Pole (N) and a South Pole (S), for clarification purposes it is noted that the "N" and "S" designations utilized in FIGS. 1-3 indicate the pole facing the annular cavity.)
More specifically, an annular magnet 22 of a certain polarity, shown as North (N) in FIG. 1, is mounted on the inner wall of the shell 14 adjacent to the spacer 16. An annular magnet 24 of a polarity opposite to that of magnet 22, i.e. South (S) in FIG. 1, is mounted on the inner wall of the shell 14 in proximity to the open end of the cavity 18. Magnet 24 is spaced apart from and is one-half the length of magnet 22. A third annular magnet 26 of a polarity opposite to that of magnet 22 is mounted on the outer wall of the core 12 in proximity to the spacer 16. Magnet 26 is the same length as and is mounted in longitudinal correspondence with magnet 22. A fourth annular magnet 28 of a polarity opposite that of magnet 24 is mounted in spaced apart relation from magnet 26 on the outer wall of the core 12 in proximity to the open end of the cavity 18. Magnet 28 is the same length as and is mounted in longitudinal correspondence with magnet 24.
Thus, magnets 22 and 26 define an "inner" pair of magnets for the first actuator, while magnets 24 and 28 define an "outer" pair of magnets for the first actuator.
As further shown in FIG. 1 a second set of magnets, similar to the first set, is mounted within an annular cavity 20 of what is illustrated in FIG. 1 as a second "left-hand" actuator. The second set of magnets includes an annular magnet 30, of opposite polarity to that of magnet 22, which is mounted on the inner wall of the shell 12 adjacent to the spacer 16. An annular magnet 32 of a polarity opposite to that of magnet 30 is mounted on the inner wall of the shell 12 in proximity to the open end of the cavity 20. Magnet 32 is spaced apart from and is one-half the length of magnet 30. An annular magnet 34 of a polarity opposite to that of magnet 30 is mounted on the outer wall of the core 12 in proximity to the spacer 16. Magnet 34 is the same length as and is mounted in longitudinal correspondence with magnet 30. An annular magnet 36 of opposite polarity to that of magnet 32 is mounted in spaced apart relation from magnet 34 on the outer wall of the core 12 in proximity to the open end of the cavity 20. Magnet 36 is the same length as and is mounted in longitudinal correspondence with magnet 32.
Thus, magnets 30 and 34 define an "inner" pair of magnets for the second actuator, while magnets 32 and 36 define an "outer" pair of magnets for the second actuator.
The arrangement of magnets in the manner shown in FIG. 1 results in the definition of three magnetic circuits. A first magnetic circuit is defined by magnets 22, 24, 26 and 28. Similarly, a second magnetic circuit is defined by magnets 30, 32, 34 and 36. Additionally, the "inner" magnets of the two above-defined sets, i.e magnets 22 and 26 of the first set and magnets 30 and 34 of the second set, interact to provide a third magnetic circuit. That is, a third "interleaved" magnetic circuit is defined by the interaction of magnets 22, 26, 30 and 34. The flux lines of the third, interleaved magnetic circuit also pass through the core element 12 and the shell 14 and the two air gaps such that the core 12 and the shell 14 carry only one-third of the total flux, thereby reducing the flux of the first two magnetic circuits.
FIG. 2 shows a double-ended moving coil linear compressor which utilizes the above-described magnetic circuit arrangement. Like elements in FIGS. 1 and 2 are similarly identified.
In the FIG. 2 embodiment, the material used for each of the magnets is Neodymium-Iron-Boron. The core 12 and the shell 14 are formed from cold rolled steel. The non-ferromagnetic material is that conventionally utilized in this type of device, e.g. type 300-series stainless steel or aluminum.
As shown in FIG. 2, in addition to the magnetic circuit arrangement shown in FIG. 1, the double-ended moving coil linear compressor further includes a coil assembly 38 which is movably disposed within the air gap of the first actuator. The coil assembly 38 includes a first coil winding 42 and a second coil winding 44 which is spaced apart from the first winding 42. Both winding 42 and 44 are connected to an appropriate electrical power supply. Winding 44 is twice the length of winding 42, the lengths and spacing of windings 42 and 44 corresponding to the lengths and spacing of the corresponding inner and outer pairs of magnets 24, 28 and 22, 26, respectively. Windings 42 and 44 are wound on the assembly 38 so that current flow in the two windings is in opposite directions to correspond to the polarities of the associated magnets 24, 28 and 22, 26, respectively. A first piston 46, which is attached to coil assembly 38, is slidably mounted within a piston chamber 48 formed in the core 12. A discharge port 49 provides fluid communication between the piston chamber 48 and the external environment through the core wall, spacer 16 and shell 14.
As further shown in FIG. 2, a second coil assembly 50, which is identical to the coil assembly 38 described above, is movably disposed within the air gap of the second actuator. The coil assembly 50 includes a coil winding 54 and a coil winding 56 which is spaced apart from winding 54 and is twice its length, the length and spacing of windings 54 and 56 corresponding to the lengths and spacing of the inner and outer pairs of corresponding magnets 30, 34 and 32, 36, respectively. Both windings 54 and 56 are connected to an appropriate electrical power supply. Windings 54 and 56 are wound on the assembly 50 so that current flow in the two windings is in opposite directions to correspond to the polarities of the associated magnets 32, 36 and 30, 34 respectively. A piston 58, which is attached to coil assembly 50, is slidably mounted within the piston chamber 48.
Thus, when current flow in the coil windings 42, 44 and 54, 56, magnetic fields are created in interact with the fields generated by the corresponding magnetic circuits, causing linear motion of the coil assemblies 38 and 50 with attendant reciprocating motion of pistons 46 and 48, respectively.
An alternative "single-ended" embodiment of a linear actuator which utilizes the above-described concepts is shown in FIG. 3.
The single-ended moving coil linear actuator 100 shown in FIG. 3 comprises a core 102 and a shell 104 which is disposed around the core 102 to define an annular space between the inner wall of the shell 104 and the outer wall of the core 102. A wall 106 of magnetic material is formed between the inner wall of the shell 104 and the outer wall of the core 102 to define an annular cavity 108 having a closed end adjacent the magnetic wall 106 and an open end. A set of magnets is mounted within the annular cavity 108 to define an air gap. A first annular magnet 110 of a certain polarity is mounted on the inner wall of the shell 104 in proximity to, but spaced apart from the magnetic wall 106. A second annular magnet 112 of opposite polarity to that of the first magnet 110 is mounted on the inner wall of the shell 104 in proximity to the open end of the cavity 108. The second magnet 112 is spaced apart from the first magnet 110. The length of the first magnet 110 is twice that the second magnet 112; that is, magnet 110 comprises two-thirds of the total length of the two magnets 110, 112 while magnet 112 comprises one-third of the total length. A third annular magnet 114 of the same polarity as magnet 112 is mounted on the outer wall of the core 102 in proximity to, but spaced apart from the magnetic wall 106. Magnet 114 is the same length as and is mounted in longitudinal correspondence with magnet 110. A fourth annular magnet 116 of the same polarity as magnet 110 is mounted on the outer wall of the core 102 in proximity to the open end of the cavity 108. Magnet 16 is spaced apart from magnet 114; it is the same length as and is mounted in longitudinal correspondence with magnet 112.
The single-ended moving coil linear actuator 100 shown in FIG. 3 further includes a coil assembly 118 which is movably disposed within the air gap 108. The coil assembly 118 includes a first coil winding 120 which is longitudinally disposed in the air gap between the first magnet 110 and the third magnet 114. A second coil winding 122, which is spaced apart from the first coil 120, is longitudinally disposed in the air gap between the second magnet 112 and the fourth magnet 116. Winding 120 is twice the length of winding 122, the lengths and spacing of the windings 120 and 122 corresponding to the lengths and spacing of the corresponding inner and outer pairs of magnets 110, 114 and 112, 116, respectively. Windings 120 and 122 are wound on the assembly 118 so that current flow in two windings is in opposite directions.